The present invention relates to fan noise cancellers and, more particularly, to a fan noise canceller, which is applicable to all fans as sources of noise, such as cooling fans for home electric products and office appliances and also air conditioner fans, and adopts an active system.
Fan noise muffling techniques are roughly classified into those of a passive system, which use sound absorbers and sound insulators, and those of an active system, which positively generate a sound wave in the opposite waveform relation to the fan noise and muffle the fan noise by sound wave interference.
A prior art active noise cancellation system for fan noise canceller is shown in FIG. 8.
As shown, this fan noise canceller comprises a first microphone 51 disposed in a duct 100 at a position near a fan 50, a second microphone 52 disposed in the duct 100 and at a predetermined distance from the first microphone 51, and a muffling loud-speaker 53 disposed mid way between the microphones 51 and 52.
The fan noise canceller further comprises a controller 54 for controlling the sound wave for cancelling the fan noise outputted from the cancelling loud-speaker 53 according to input signals from the first and second microphones 51 and 52.
In this fan noise canceller, a sound wave which is generated from the fan 50 as a source of noise and propagated through the duct 100 is detected by the first microphone 51 and coupled to the controller 54. At this time, a signal from the second microphone 52 which evaluates the cancelling effect is also coupled to the controller 54.
The second microphone 52 for evaluating the cancelling effect, detects a sound wave that results from the interference of the sound wave generated from the cancelling loud-speaker 53 and the sound wave propagated from the fan 50. The controller 54 drives the cancelling loud-speaker 53 by generating, in a digital signal processing or like process, a signal for making the signal from the second microphone 52 to be zero. This has an effect of reducing noise at the position, at which the second microphone 52 is disposed.
The prior art fan noise canceller as shown above has an advantage that it can be installed after the installation of the fan 50. It also has an advantage that it cancels noise on the side of the second microphone 52 (i.e., adjacent the duct end from which air is sent out), and the operation thus is not readily affected by the noise characteristic changes or system changes in long use.
In the above prior art fan noise canceller, however, the first microphone 51 and the cancelling loud-speaker 53 are disposed such as to form a closed loop as an electrical-acoustical system. Therefore, the operation of the controller 54 readily becomes unstable, and sometimes hauling occurs to increase the noise.
In addition, in the prior art fan noise canceller a harmonic wave is generated by detecting the number of rotations of the fan. Therefore, a predetermined time is required for the signal processing that is necessary for generating the opposite waveform sound wave. For this reason, this fan noise canceller is unsuitable for a fan which does not have any duct, although it is suitable for the fan with the duct because a predetermined distance is provided between the fan and the cancelling loud-speaker. This means a disadvantage of the prior art fan noise canceller in that it is necessary to provide the duct or the like.